This project is a continuation of an analysis of primate motor control systems in which the primary emphasis is on motor cortex modulation of the spinal motor control system. Currently we are analyzing muscle afferent projections from hindlimb musculature to precentral neurons in areas 4 and 3a. Of particular concern is the measurement of position and velocity sensitivities of precentral neurons. This is accomplished by recording cortical unit discharge at different rates and amplitudes of muscle stretch. The influence of muscle afferent input on cortical neurons whose axons descend in the lateral corticospinal tract is also under investigation.